Valleylab Force FX-8C, Service Manual

Page: 71 / 218

Footswitch Board
Principles of Operation
Force FX-8C Service Manual 4-15
• When COM0 is selected, the power control switches may be read.
• When COM1 is selected, the recall switch and the bipolar mode select
switches may be read.
• When COM2 is selected, the cut and coag mode select switches may be
read.
To read the switches, the main microcontroller asserts the desired BANK
select line and reads the state of the keyboard switch return lines KBD_D0
through KBD_D7. These lines are read through port A of the I/O
expansion port (U2) on the Control board.
Footswitch Board
Refer to Section 9 for components and the Schematics Supplement for
Board Drawings and Schematics.
The Footswitch board is mounted inside the rear panel. It contains
circuitry accepting and decoding footswitch keying inputs and an audio
circuit for announcing generator keying and various alarm tones. The
Footswitch board interfaces with the Power Supply/RF board.
Footswitch Decode Circuit
Two monopolar footswitch connectors and a bipolar footswitch connector
are mounted on the Footswitch board and extend through the rear panel.
The monopolar footswitch connectors (J3 and J2) accept monopolar
footswitches and provide footswitching capability for the Monopolar 1/
CEM and Monopolar 2 Instrument receptacles located on the front panel.
The bipolar footswitch connector (J4) accepts a bipolar footswitch and
provides footswitching capability for the Bipolar Instrument receptacle on
the front panel. Capacitors C39–C46 provide filtering that blocks high
frequency noise from exiting the generator on the footswitch cables.
As required by the IEC, the footswitch circuit is isolated from patient
connected and ground referenced circuits and is able to withstand a
potential of 500 Vrms (50/60 Hz). To obtain this isolation, the footswitch
connected circuitry is powered from an isolated power supply (U2). The
isolated power supply, an HPR-107, operates from the ground referenced
+12 V power supply and supplies an isolated 12 volts.
Resistors R18 and R19 form a voltage divider that yields a signal (Vref2) of
approximately 6 volts. This reference voltage is applied to the
noninverting inputs of comparators U3A, U3B, U4A, U4B, and U5A. The
common terminal of each footswitch is connected to the +12 V isolated
power source. Footswitch activation causes this voltage to be applied to a
resistor divider. The values of the resistors comprising the input divider
are selected to provide a switching threshold of approximately 750 ohms.
The divided voltage is then applied to the inverting input of one of the
five comparators. When the voltage at the inverting input exceeds the
voltage at the noninverting input, the open collector output of the
comparator turns on, causing current to flow in the LED of the
corresponding optoisolator. This current generates an IR beam that causes